1. Field of the Invention
The present invention relates to an element for detecting a target substance, which is useful for detecting the presence or absence of the target substance in a specimen; a base carrier for preparing the same; a detection method with the use of the element; and a kit for detecting the target substance.
2. Description of the Related Art
Conventionally, a measurement method has been known which includes the steps of: preparing a metallic fine particle on which an antibody or an antigen has been previously adsorbed; making the antigen or the antibody cause an antigen-antibody reaction; and observing a change of a spectrum before and after the reaction by using surface plasmon resonance. For instance, Japanese Patent Application Laid-Open No. H07-146295 discloses a method of making a metallic fine particle previously adsorb an antibody corresponding to an antigen of an object to be detected on its surface, and using the metallic fine particle for an antigen analysis. The method includes the steps of: adding the antigen of the object to be measured to the fine particles which have adsorbed the antibody, to cause an antigen-antibody reaction; measuring a Raman spectrum in “a state of a fine particle having an immune complex adsorbed on its surface due to the antigen-antibody reaction”; identifying the formed immune complex; and furthermore, quantifying the added aiming substance on the basis of the intensity of Raman scattered light.
R. P. Van Duyne (J. Phys. Chem. B. 2001, 105, 5599-5611) and F. Hook (Anal. Chem. 2004, 76, 7211-7220) propose a method for improving detection sensitivity in comparison with a method of using a metallic fine particle as an element, by using surface plasmon resonance on an element composed of a thinned film having a nanostructure.
On the other hand, a biosensor to which the present invention is applicable, is a measurement device that makes use of an excellent biomolecule recognition capability of a living body and a biomolecule, and has been expected to be applied to a wide range of not only a medical field but also an environment field or a foodstuffs field, in recent years.
A biosensor generally includes: a capturing body for recognizing and capturing a substance to be measured (hereafter referred to as a target substance); and a detecting element which senses a physical or chemical change occurring after capturing, and converts the change into a detectable signal such as an electrical signal and a light signal. In a living body, there are several combinations of substances having an affinity for each other, such as the combinations of enzyme-substrate, antigen-antibody and DNA-DNA. The biosensor makes use of a principle that one substance of the combination can be selectively measured by immobilizing or carrying the other substance of the combination on a substrate and using the other substance as a capturing component. In addition, various types of detecting elements have been proposed which include an oxygen electrode, a hydrogen peroxide electrode, an ion electrode, ISFET and a thermistor. Recently, even a crystal resonator or an SAW element has been occasionally used, which can sense a change of mass in a nanogram order.
A measuring method using surface plasmon resonance has such characteristics as to merely need a simple structure for an assay because of needing no labeled-molecule such as a fluorescent dye, and be capable of directly monitoring a reaction process in which an object is being adsorbed by a metallic surface, in real time. For this reason, the measuring method using the surface plasmon resonance is expected to be applied to various assays. However, when a conventional element for detecting a target substance with the use of the surface plasmon resonance, such as an element using a fine particle, is used for an affinity assay such as immuno-assay which makes use of the specificity of an antigen-antibody reaction, the element can not occasionally show sufficient detection sensitivity. In addition, an element employing a thin nanostructure formed on a flat substrate can show the sensitivity, but needs a long period of time for a detecting reaction. In other words, a problem of the detection sensitivity and the reaction time is a big subject for expanding an application range of the measuring method using the surface plasmon resonance.